1. Field of the Invention
This invention relates to an apparatus for effecting the separation of a gas mixture into a first fraction and a second fraction. More particularly, the invention relates to a vortex gas separator which creates a vortex on the interior of the apparatus and relies upon the selective interaction of the first and second fractions of the mixture with a sorbent material.
While the invention is applicable to any area in which the separation of a gas mixture into various components is desirable, the invention has particular utility in the treatment of the radioactive off-gases of a nuclear power plant. Therefore, the invention will be described with relation to its application in nuclear reactor systems, but the scope of the invention should not be so limited.
When uranium is fissioned in a nuclear reactor, certain fission products are formed. A fraction of the formed fission products consist of gaseous xenon and krypton. Certain isotopes of xenon and krypton are radioactive and constitute a danger if allowed to escape and to become distributed by a fluid or a gaseous medium.
Ordinarily the gaseous xenon and krypton isotopes are contained within the fuel rods of the nuclear reactor by the fuel rod cladding. However, in the event that leakage develops through the cladding, these fission product gases are released into the primary coolant. The reactor coolant disperses the dissolved fission gases throughout the primary coolant system including the auxiliary systems such as the Boric Acid Treatment System. Occasionally when a leak develops in the steam generator tubes, the primary coolant and the dissolved fission gases are leaked into the secondary coolant. With the dispersal of these radioactive fission products throughout the coolant systems, there are a number of locations from which they may leak into the atmosphere within the containment structure, the auxiliary building or the fuel handling building. Typical of these locations are the pump and valve seals which may leak during plant operation and the refueling pool which emits the dissolved fission gases during plant shutdown. Accumulation of the xenon and krypton in the containment atmosphere would eventually render it unsafe to enter into and work within the containment structure. It is, therefore, necessary to remove these fission products at least periodically from the coolant and from the various cover gases within the system. Environmental considerations, however, preclude the wanton dumping of restricted amounts of radioactive gases into the atmosphere. Temporary retention for radioactive decay, and disposal, therefore, is the most acceptable method for dealing with these gases. However, retention and disposal is complicated by the fact that the coolant off-gases contain large amounts of hydrogen and nitrogen with only small quantities of the radioactive fission product gases. Therefore, development of a method of concentrating the radioactive fission products by separation out of the hydrogen gas is desirable.
2. Description of the Prior Art
Various methods have been developed and put into use to accomplish the radioactive decay of fission products removed from the reactor coolant. One method involves the collecting and storage of the gases dissolved in the coolant for a period of time in gas decay tanks to permit the decay of the shorter half-life radionuclides. These gases are then periodically discharged to the atmosphere. The disadvantages of this system are that: (1) valuable carrier gases such as nitrogen must be stored and wastefully discharged to the atmosphere along with the radionuclides; (2) certain fission products, particularly Kr.sup.85, have long half-lives and their discharge to the atmosphere is not desirable even though the amount of such nuclides is small and far below the maximum permissible concentration; and (3) large storage volumes and high pressures are required which increases the expense of the off-gas treatment system.
The adsorption of noble gases on charcoal at ambient temperatures is the process that has been most extensively proposed and used in BWR's. This is also a method for delaying the release of the noble gases to the atmosphere in order to allow the short-lived isotopes (primarily xenon) to decay. However, it has certain disadvantages such as: (1) large beds of charcoal are required; (2) the charcoal burns readily; and (3) less decay time is typically available than with storage systems. If the charcoal beds are refrigerated, operating costs increase and materials that would freeze or condense must be completely removed from the gas prior to its injection into the bed in order to prevent plugging of the equipment.
In cryogenic separation, another process that has been proposed for noble gas retention, the noble gases and part of the air, or other carrier gas, are first liquified. Then the noble gases are separated from the bulk gases and are concentrated by fractional distillation. As in all of the low-temperature operations, water and other gases that would form solids must be essentially completely removed prior to the treatment of the noble gases. Solids in the system cause physical difficulties, and the presence of liquid ozone, which is formed from the radiolysis of oxygen, creates an explosion hazard.